1. Field of the Invention
This invention relates to a device for converting incident wide bandwidth short duration individual pulses of electro-magnetic radiation, in particular individual pulses with bandwidths greater than 1 GHz and durations of an order of or less than 10 microseconds, into forms that can be more readily digitally sampled.
2. Discussion of Prior Art
Digital sampling of an analogue signal usually employs analogue-to-digital convertors (ADCs) and is performed to obtain a digital record of the signal, in a form suitable for storage, analysis, manipulation by computer or display on a digital oscilloscope, etc. For an analogue signal of bandwidth B to be sampled sufficiently, ie. so that the analogue signal can be reproduced from the digital record without distortion, the sampling rate should be equal to or exceed the Nyquist sampling rate of 2 B. For signals of low bandwidth, for example, audio signals it is not difficult to sample a waveform continuously at the required rate, which is of the order of 10 kHz. The ADCs used for such tasks may be of the "successive approximation" type, which provide great accuracy at low sampling rates. For wider bandwidth signals, which require higher sampling rates, it becomes necessary to use the more complicated and expensive "flash convertor" technology in which the incoming signal is compared with many reference voltages in parallel. For pulses whose bandwidth is greater than around 1 GHz there are no ADCs available at the present time which can operate at the required sampling rate.
For repetitive pulses with bandwidths greater than 1 GHz it is possible to obtain a digital record of a single pulse by taking successive or quasi-random samples on successive pulses at a sampling rate lower than 2 B in order to eventually build up a replica of the repeated pulse. This is a procedure which is commonly used in commercial measurement equipment, for example, digital oscilloscopes. However, this does not solve the problem of sampling an individual, short duration pulse with a bandwidth greater than 1 GHz which must be digitised in a single acquisition cycle.
The developers of digital oscilloscopes have attempted to tackle this problem by conducting research to produce faster sampling methods, ie. faster ADCs. However, this approach incurs large development costs and leads to state of the art hardware being used which naturally is very expensive. This research has enabled (at the present time) sampling rates of up to around one gigasample per second to be achieved, but at an accuracy of only a few bits. A sampling rate of one gigasample per second is still not great enough to digitally sample the short duration large bandwidth individual pulses discussed above.